Generally, the radionavigation signals transmitted by the satellites (or pseudolites) of a positioning system take the form of a carrier modulated by a spreading waveform containing a pseudo-random binary code. Since modulating the carrier causes spreading of the spectrum around the frequency of the carrier, radionavigation signals are frequently referred to as “spread-spectrum”. The pseudo-random codes constitute an identifier of the signal and therefore of the transmitting satellite. Known to the receivers, said codes provide the receivers with Code Distribution Multiple Access (CDMA). Incidentally, certain satellite positioning signals may also carry useful data (for example the navigation message) in the form of a binary sequence (at a substantially lower rate than the pseudo-random code) additionally modulated on the carrier. This payload of useful data will be disregarded hereafter.
In the case of GPS, radionavigation signals are transmitted in the L1 frequency band, centered on 1575.42 MHz, the L2 frequency band, centered on 1227.6 MHz and the L5 frequency band, centered on 1176.45 MHz. The satellites of the European GNSS (also known as “Galileo”) will transmit in the bands: E2-L1-E1 (the median band portion L1 being the same as that for GPS), E5a (which, according to Galileo nomenclature, is the L5 band provided for GPS), E5b (centered on 1207.14 MHz) and E6 (centered on 1278.75 MHz). Hereafter, the E5a and E5b bands will be treated together as the E5 band, with 1191.795 MHz as the central frequency. In the case of Galileo's open signals, a complete description may be found in “Galileo Open Service Signal-In-Space Interface Control Document”, or Galileo OS SIS ICD, available on the website http://ec.europa.eu/enterprise/policies/satnav/galileo/open-service/Index_en.htm. It may also be noted that the satellites of the Compass constellation transmit or will transmit in band B1 (centered on 1561.098 MHz), B1-2 (centered on 1589.742 MHz), L1 (centered on 1575.42 MHz), B2 (centered on 1207.14 MHz) and B3 (centered on 1268.52 MHz). The GLONASS system uses the central frequencies 1602 MHz and 1246 MHz. The stated central frequencies are the frequencies of the carriers of the various signals.
Receiving a radionavigation signal normally involves multiplying the received signal by an internal replica of the carrier generated in the receiver by an oscillator driven by a carrier tracking loop and another multiplication by an internal replica of the spreading waveform produced by a waveform generator driven by a spreading waveform tracking loop (also known as a “code tracking loop”). The error or servo signals of the carrier and spreading waveform tracking loops are used by the receiver to determine its position. The signal representing the phase difference between the carrier of the received signal and the internal carrier replica produced at each time step by the carrier tracking loop provides a first measurement or observable (the phase measurement or observable). The time offset signal between the spreading waveform of the received signal and the internal spreading waveform replica produced at each time step by the spreading waveform tracking loop represents a second measurement or observable (the code measurement or observable).
The phase observable is not used by all receivers. Inexpensive receivers in particular determine their position solely on the basis of code observations. Phase measurements are implemented, for example, in the RTK (“Real Time Kinematic”) and PPP (“Precise Point Positioning”) methods.
Code measurements have meter-level accuracy while phase measurements have an accuracy of a few mm. However, phase measurements have the major drawback of being ambiguous in that the number of integer cycles between the satellite and the receiver is unknown at the outset. Phase measurements are modulo one cycle and only provide the real part of the carrier phase difference between transmission by the satellite and the receiver. In order to be able to benefit from the accuracy of phase measurements, a receiver must be able to resolve the ambiguities associated therewith.
Carrier phase tracking of a radionavigation signal is very sensitive to environmental conditions. The risk of dropout is much higher than for code tracking. Furthermore, managing cycle slip is a difficult task. In a challenging environment (for example in an urban area), availability of phase measurements is likely to be very low, so making receivers capable of carrying out and processing phase observations very much less worthwhile.